In the recent years, as a technique to enable a high capacity data communication, a MIMO (Multiple-Input Multiple-Output) system is attracting attention. In the MIMO system, a radio transmission apparatus spatially multiplexes and sends transmission data (stream) from each of a plurality of transmitting antenna ports, and a radio receiving apparatus receives receiving data in which a plurality of transmission data is mixed by spatially separating it into original transmission data on a transmission path (e.g. see non-patent literature 1).
In the MIMO system, in a case where the number of transmitting antenna ports is Ntx and the number of receiving antenna ports is Nrx, the number of streams that can be multiplexed can be obtained by following equation 1.
[1]min(Ntx,Nrx)  Equation 1
Here, function min(x, y) is a function that returns a smaller value of x and y. That is, the number of streams that can be multiplexed is a smaller one of the number of transmitting antenna ports Ntx and the number of receiving antenna ports Nrx.
Note that, in the MIMO system, in a case where a transmission bandwidth is broadened by an higher data transmission rate and a multipath occurs, typically, a multipath environment of each stream is processed by being converted to a single-path environment (i.e. the number of paths being 1) by using an OFDM (Orthogonal Frequency Division Multiplexing), etc.
Further, as an extension scheme of the MIMO system, in a case where the number of transmitting antenna ports is greater than the number of receiving antenna ports, a scheme in which the number of streams is increased by using the multipath generated on the transmission path is proposed (e.g. see non-patent literature 2).
Specifically, in a case where the number of transmitting antenna ports is Ntx, the number of receiving antenna ports is Nrx, and the number of paths is M, the number of streams that can be multiplexed can be obtained by following equation 2.
[2]min(Ntx,Nrx×M)  Equation 2